Scientists reveal secret of a cat's lap

Engineers have used high-speed videos and mechanical gizmos to figure out the mechanics of a cat's drinking style, confirming what most pet owners know already: Cats are way different from dogs.

Dogs drink by dipping their tongues into liquids like ladles. A little pool of water is brought into the mouth every time Fido takes a gulp from the toilet bowl. Cats, in contrast, touch only the curled-back tip of the tongue onto the surface of the liquid. When the tongue is drawn back up into the cat's mouth, a thin column of the liquid is drawn up as well. Then the cat closes its mouth around that column.

Roman Stocker, an engineering professor at the Massachusetts Institute of Technology who specializes in fluid dynamics, admitted in a Science video interview that the research was "somewhat unusual."

M. Pilotto, R. Stocker, P. Reis

How do cats lap? Cutta Cutta shows how it's done.

"What prompted this study of cats was in fact my cat," he said. "I was watching him one morning, three years ago over breakfast, as he was lapping his water. It occurred to me that there was an exciting biomechanics problem hidden behind the commonplace and apparently simple action of lapping."

Researchers spent hours at Stocker's house, waiting for the cat (named Cutta Cutta) to take a drink while the high-speed video camera was rolling. They also analyzed scores of videos of drinking cats -- ranging from recordings of big cats at Zoo New England to cute videos they found on YouTube.

They found that domestic cats average about four laps per second, with each lap bringing in about 0.1 milliliters of liquid. In contrast, tigers, lions and jaguars lap at less than half the rate. In each scenario, the lapping action strikes a balance between the inertia that makes the liquid rise into the cat's mouth ... and the gravity that makes the liquid fall.

"What is remarkable is that cats seem to know about this balance, and lap with a frequency that maximizes this volume ingested," said MIT's Pedro Reis, another co-author of the paper.

To tease out the full story behind a cat's lap, the researchers basically used an artificial cat tongue -- a little glass disk attached to the end of a plunger that dipped down onto the surface of a liquid at an adjustable rate. This gave the scientists a way to quantify how gravity and inertia worked together ... without waiting for Cutta Cutta to decide he was thirsty.

This is the kind of research that sounds as if it came to a conclusion everybody already knows, but Stocker told me these are new discoveries. "I don't think anyone has explained it," he said, "certainly not the balance of forces behind it."

A lapping cat was one of the subjects covered in the Oscar-winning short subject titled "Quicker 'n a Wink," released back in 1940. MIT engineer Doc Edgerton recorded the cat's technique using a high-speed camera, but didn't delve into the detailed fluid mechanics behind it.

Stocker admitted that there's not an immediate practical application to the research, which was conducted with borrowed equipment and no outside funding. But he said the mechanics of a cat's tongue could be adapted for robotic devices that move water around using soft structures (like, say, an elephant's trunk). There might also be some evolutionary lessons to be learned from the differences between the drinking habits of cats and dogs.

The way dogs drink liquids is somewhat messier, Stocker noted. "Part of a cat's face is highly sensitive -- for example, the whiskers. There might be a desire on the part of the cat to keep those dry," Stocker speculated.

But when you get right down to it, the experiment was "purely curiosity-driven," Stocker said.

"Although people may not appreciate that, curiosity is often what motivates science," he said. And the nicest part is that in this experiment, curiosity did not kill the cat.

"The cat is still quite happy," Stocker told me over the telephone. "In fact, he's sitting right beside me now."

Update for 2 p.m. ET Nov. 12: Over at the Not Exactly Rocket Science weblog, Ed Yong goes into more detail about the experiment as well as feline drinking habits. He includes a link to "Quicker 'n a Wink" on YouTube -- a link that eluded me yesterday. You can see the cat-drinking footage at around the 4:40 mark in this clip: